Methylmercury (MeHg) is a persistent environmental toxin that preferentially targets the developing nervous system. MeHg exposure is of great concern for human health due to its presence in dietary fish. However, the risks of MeHg exposure commonly encountered today remain uncertain, largely due to the variable outcomes observed among individuals experiencing similar levels of MeHg exposure. Our long-term objective is to resolve the molecular and genetic mechanisms that control MeHg toxicity and thereby contribute to more scientifically sound risk assessment. Our overall aim is to investigate a genetic component for susceptibility and tolerance to MeHg. In a screen for genes that confer tolerance to MeHg during development in Drosophila, we have identified cytochrome p450 (CYP) family members as candidates. CYPs are Phase I metabolism enzymes that modify xenobiotics in detoxification pathways and perform essential activating reactions for endogenous and exogenous biomolecules. We found CYP6g1 is the most highly expressed and regulated CYP in MeHg-tolerant flies and that over-expression of CYP6g1 by transgene or via a naturally occurring high-expressing allele confers MeHg tolerance. We therefore propose a novel hypothesis that predisposition to MeHg toxicity is influenced by innate levels of CYP activity. With this proposal we intend to improve scientific knowledge by translating our findings in flies to humans. Our two Specific Aims are designed to investigate the functional and genetic association of the human homologs of CYP6g1 (CYP3A4, CYP3A5 and CYP3A7) with tolerance to MeHg toxicity in people. CYP3A members are the most highly expressed CYPs in the human liver. CYP3A7 and CYP3A4 are distinguished by being the predominant fetal and adult CYPs, respectively. In addition, CYP3A4, CYP3A5 and CYP3A7 polymorphisms and haplotypes exist that are predicted to be more and less active, and which show high frequency Africans versus Caucasians, respectively. In Aim1 we will determine the relative activity of each of the CYP3A isozymes in conferring MeHg tolerance during development. This will be accomplished using our established MeHg-tolerance assay in transgenic Drosophila. In Aim2 we will probe for the association of CYP3A genotypes and MeHg toxicity outcomes in two established cohorts of MeHg-exposed children that are predominantly African or Caucasian. Our CYP single nucleotide polymorphism (SNP) probe analyses will be done as an addendum to an ongoing large-scale genotyping project with these two cohorts. Association of MeHg outcomes with CYP genotype will be resolved through multivariate analyses of CYP alleles with consideration of potential confounders. Results from this study will have a potentially lasting impact by identifying a genetic component of variability in susceptibility to MeHg in developing children. Identification of CYPs as genetic markers of susceptibility to MeHg would greatly advance risk assessment by bringing new technical abilities to health care providers for identifying and advising at-risk individuals in a clinical setting.